Artificial graphite electrodes are produced by pulverizing a coal coke or a petroleum coke and classifying it to a fixed particle size distribution and then subjecting it to kneading with a binder pitch, extrusion-molding, baking, impregnation, secondary baking, and graphitization. In a graphitization step, calcined electrodes are graphitized at about 3000.degree. C., wherein an Acheson furnace has so far mainly been used. In recent years, however, an LWG furnace (direct current flow type) has come to be mainly used. Since graphitization in this LWG furnace accelerates a temperature elevating rate, a generating rate of gas is increased, and therefore an abnormal expansion phenomenon called puffing is liable to take place. Puffing not only lowers a density of an electrode but also sometimes damages the electrode when the phenomenon is severe. Accordingly, processes for producing a needle coke having reduced puffing and puffing inhibitors added in producing electrodes have so far been investigated.
It is estimated that a puffing phenomenon takes place due to sulfur volatilized at temperatures of 1700.degree. to 200.degree. C. at which a graphite structure starts to be formed. Accordingly, iron oxide which reacts with sulfur to form sulfide to change the timing for the volatilization of sulfur is usually used as a puffing inhibitor (Japanese Patent Application Laid-Open No. Sho 55-110190). Further, various compounds as well as nickel compounds (Japanese Patent Application Laid-Open No. Sho 60-190491), titanium oxide (Japanese Patent Application Laid-Open No. Hei 2-51409) and the like have recently been proposed.
Further, as a process for a producing needle coke having puffing reduced without depending on a puffing inhibitor, a process is proposed in which pitch from which quinoline insoluble matters are removed is subjected to hydrorefining in the presence of a hydrogenating catalyst and then turned into a coke to thereby remove a nitrogen content or a sulfur content contained in the raw material pitch, which is an original substance causing puffing (Japanese Patent Publication No. Sho 59-122585).
On the other hand, a process is proposed as well in which a crude coke is subjected to a first stage baking at lower temperatures than ordinary calcining temperatures, for example, about 800.degree. C., and after this is once cooled down, it is subjected again to a second stage calcination at temperatures falling in a range of 1200.degree. to 1500.degree. C. (Japanese Patent Publication No. Sho 53-35801).
Among these techniques, all of those depending on puffing inhibitors use the puffing inhibitors added in a step for kneading a coke with a binder pitch, and the effect thereof is varied depending on the kind of a coke used as a raw material For example, iron oxide has a defect that while it shows an excellent effect to a petroleum coke, it shows only a small effect to a coal coke. On the other hand, the processes which do not depend on puffing inhibitors have a problem that they have a difficulty in profitability and therefore have not yet been put to actual use or do not necessarily provide a satisfactory effect of reducing puffing.
Intensive researches made by the present inventors in order to solve the problems described above have resulted in finding that puffing caused in producing electrodes is reduced by adhering to a coke a substance having a softening point or a melting point which does not exceed 1800.degree. C. after heating to 200.degree. C. or higher, in producing a needle coke for a raw material of a graphite electrode, particularly boron compounds, and thus completing the present invention.